There are two types of widely used fluxing technology in the electronics industry: liquid spray and liquid foaming. For both techniques, to achieve a uniform deposition, the solvent in the flux must wet the electronic board to be soldered to form a continuous film. Conventionally, the volume of the solvent needed to wet a board is relatively large. But for a soldering operation having no post-soldering cleaning steps, the flux residue after the soldering must be benign and minimum in quantity. Accordingly, most no-clean fluxes usually have very high solvent content (95% to 99%) and low solid content (typically 1% to 5%).
There are basically two types of solvents in use today: volatile organic compounds (VOC) which evaporate easily during the soldering process, and water. Low solid content fluxes using alcohol or other organic compounds as solvents wet the board easily. However, they emit large amounts of VOC during the soldering process and thus create environmental problems. Low solid content water-based fluxes, on the other hand, need a surfactant to assist wetting, since water has a very high surface tension when deployed upon the electronic board and metals to be joined. The surfactant leaves a hygroscopic residue after the soldering operation and thus has to be cleaned off or the final product has to be protected with a conformal coating or an encapsulant.
The quest for better ways to clean precision electronic components without ozone-depleting solvents had led to the development of cleaning processes that reduce the need for solvents. Techniques have now emerged for using supercritical carbon dioxide instead of environmentally harmful CFC-based solvents to remove particles and organic contaminants introduced during the manufacturing of circuit boards. However, the use of a supercritical carbide dioxide may tend to adversely attack the board itself or a plastic housing which may accommodate the board. Accordingly, for these among other reasons, supercritical carbon dioxide may be a sub-optimal approach to cleaning electronic circuit boards.
U.S. Pat. No. 5,447,577 (issued to the assignee of the present application) disclosed that heating a water solution of adipic acid dramatically increases the solubility of adipic acid in water (col. 4, line 26). As a result, a uniform flux deposition can be achieved by using only a small fraction of the solvent used in conventional low-solid fluxes (col. 4, line 12). The minimal quantity of solvent can eliminate the need for a surfactant that leaves highly hygroscopic residue. Thus, post-soldering cleaning or the need for a conformal coating or an encapsulant is avoided (col. 4, line 23). Thus, the disclosure of the '577 patent focused on the use of supercritical carbon dioxide (claims 1-14) as the solvent and carrier for the flux. Claims 15-18 disclosed the use of carbon dioxide in a low pressure state as a stream separate from the flux.